1. Field of the Invention
The present invention relates to an exhaust gas purification system for trapping carbon and other particulates in an exhaust gas of a diesel engine. More particularly, the invention relates to a particulate trap system for an exhaust gas, by which a filter for trapping particulates, such as a ceramic filter, can be regenerated.
2. Description of the Related Art
In an exhaust passage of a diesel engine, an exhaust gas purification system or an emission control system is provided for trapping particulates, such as carbon particles and so forth, as an anti-pollution measure. An example of such an exhaust gas purification system is illustrated in FIG. 7.
In FIG. 7, a particulate trap system 14 is connected to an exhaust pipe 12 of a diesel engine 10, and a particulate trap filter 16 is disposed inside the particulate trap system 14. The particulate trap filter 16 is formed as a porous ceramic cylinder having a honeycomb structure, and defines a plurality of upstream side passages 16a and downstream side passages 16b separated by porous partitions 18, as shown in FIG. 8. The downstream ends and the upstream ends of the upstream side passages 16a, and the downstream side passages 16b, are closed respectively, and accordingly, the exhaust gas from the diesel engine 10 flows into the purification system 14 through upstream side open ends of the upstream side passages 16a. The gaseous component of the exhaust gas then passes through the porous structure of the porous partitions 18 into the adjacent downstream side passages 16b, and is then subsequently discharged. The particulates, such as carbon particles, contained in the exhaust gas are blocked by the partition 18, and thus are trapped and accumulated in the upstream side passages 16a.
Nevertheless, an increase of the amount of accumulated particulates causes an increase in the resistance to the exhaust gas flow, to thus increase the pressure difference .DELTA.p between the upstream end and the downstream end of the particulate trap filter 16, which may lower the output of the engine 10. Therefore, it is necessary to periodically remove the accumulated particulates, and accordingly, an electronic heater 20 is provided on the upstream side wall surface, for heating and burning the trapped particulates, to thereby regenerate the particulate trap filter.
In FIGS. 7 and 8, 22 denotes a filter casing forming the outer shell of the purification system 14, 24 denotes a bypass passage for allowing the exhaust gas to bypass the purification system 14, and 26 denotes a bypass valve for selectively switching the exhaust gas flow path.
During the regeneration process, heat generated by the burning of the particulates in the vicinity of the outer periphery of the particulate trap filter 16 can escape to the atmosphere, through the filter casing 22, and this can cause a lowering of the temperature of the particulates to lower than the burning temperature thereof, and thus unburnt particulates remain and the regeneration treatment is only partially successful.
FIG. 9 shows local temperature variations during the regeneration treatment, to represent the above-mentioned condition of remaining unburnt particulate. In FIG. 9, the solid line shows temperature variations according to a processing time at the center portion A of the particulate trap filter 16 (for example, in the region A in FIG. 8), and the broken line shows temperature variations according to the processing time at the outer circumferential portion B away from the center (for example, at the region B in FIG. 8). Due to the increase in the difference (temperature difference .DELTA.T1) between the peak values of the two curves, the amount of unburnt particulates at the outer circumferential portion B is increased. Also, when the temperature at the central portion A of the filter 16 becomes much higher than that at the outer circumferential portion B, the filter may be destroyed by a substantial thermal distortion thereof.